what is user data after a DCL EXCM?

Running MATPG on some old programs, I observe that some exception monitors
contain a "user data" portion, as if the original source code had been
written like this:

DCL  DD  THING1  CHAR(2)  AUTO  INIT(X"E000");
DCL  EXCM  EOFMON  INT(THING2)  IMD  CV("CPF")  USRDTA(THING1)
EXCID(H"5001");
ENTRY  THING2  INT;

Now I know the QPRCRTPG API won't accept this synatx, however, the CRTPG
instruction accepts the binary equivalent of a USRDTA(ODT#) value in an
exception monitor.  What does it do?  And what is the correct old MI
syntax: USRDTA, DTA, DATA ...?